The invention is generally related to the field of forming capacitors on a semiconductor device and more specifically to forming large area capacitors on high speed RF devices.
High performance capacitors are needed for RF circuits such as ac coupling capacitors in mixers and LNAs, ADCs (analog to digital converters), DAC (digital to analog converters), and filters. High intended capacitance per square micron, low resistances, and high intended capacitance/parasitic capacitance ratio are critical. Several types of capacitors are known in the art. FIG. 1A shows a poly/diffusion capacitor in which the capacitor is formed using a polysilicon layer 16 separated from a doped region 12 by a gate oxide layer 14. FIG. 1B shows a poly/poly capacitor in which a first doped polysilicon layer 20 is formed on a field oxide region 18 and separated from a second doped polysilicon layer 24 by an oxide layer 22. A third capacitor type is shown in FIG. 1C. It is a metal/metal capacitor in which two metal layers 30,34 are separated by a thicker oxide layer 32. Metal/metal capacitors have lower intended capacitance per square micron and thus require a larger area.
Two newer capacitors are shown in FIGS. 1Dand1E. These capacitors use a bottom plate comprising a layer of doped polysilicon 40 with an overlying layer of titanium-silicide 42. The top plate 46 is titanium-nitride. The capacitor dielectric 44 is an oxide. In one variation, the bottom plate polysilicon 40 is separated from a underlying layer of doped polysilicon 48 by a thicker oxide layer 50. However, further decreases in the parasitic capacitance to substrate and sheet resistance is desired.
The invention is a capacitor having a bottom plate that comprises undoped polysilicon which has been silicided. An advantage of the invention is providing a capacitor having reduced parasitic capacitance to the substrate and reduced sheet resistance of the bottom plate.
These and other advantages will be apparent to those of ordinary skill in the art having reference to the specification in conjunction with the drawings.